This application claims priority from PCT/SE97/01953 filed on Nov. 21, 1997, which claims priority from Swedish application Ser. No. 9604282-5 filed on Nov. 22, 1996.
The present invention relates to an electrode for control of an electric field in a gaseous electrically insulating medium.
Electrically conducting objects at a high electric potential relative to the surroundings give rise to high electric field strengths in their vicinity. This applies particularly to objects with a small radius of curvature and parts projecting from the object into the surrounding insulating medium.
If the electric field strength exceeds the dielectric strength E0 of the medium, in dependence on the degree of inhomogeneity of the electric field and the change of the voltage with time, this leads to electric partial discharges from the electrode surface and/or an electric breakdown through the medium to another electrically conducting object or a ground plane.
Electric breakdowns must inevitably be prevented since they entail an electric short-circuit of the equipment, where the electrodes are included.
Also partial discharges are undesirable, since, among other things, they give rise to radio interference, energy losses and chemical degradation of material in that the partial discharges generate chemically aggressive and poisonous substances such as ozone and nitric oxides in air and a number of fluorine and sulphur compounds in the technical insulating gas sulphur hexafluoride. Partial discharges, which are harmless per se, may make the measurement of partial discharges of other parts of the equipment impossible.
Commonly used methods of reducing the electric field strength in the vicinity of electrically conducting objects comprise selecting, in the entire design and dimensioning of devices intended for high electric voltages, mutual distances between electrically conducting objects and radii of curvature on the surfaces thereof, such that the dielectric strength of the medium is not attained at any place in the device.
Since such dimensioning would often lead to very large dimensions of the equipment, areas with especially high local field strengths, such as the high-voltage side of bushings or conductor bends in gas-insulated switchgear, GIS, are provided with screening electrodes which have sufficient radii of curvature to keep the electric field strength below the dielectric strength of the medium. The electric field strength E in the vicinity of an electrically conducting object is of the order of magnitude of E≈U/R, where U is a typical electric potential difference for the equipment and R is the radius of curvature. This implies that the smallest allowable radius R0 of curvature is about U/E0. The radius of curvature, in its turn, is dimensioning for the dimensions of the screening electrode, which often becomes a significant part of the dimensions of the whole device.
Large screening electrodes increase the capacitance of the screening part. For many fields of use, the increasing capacitance implies an unnecessary load on the voltage/current source. The electrostatic energy in the device which is released during a short-circuit, which, for example, is caused by an electric breakdown, increases with increased capacitance.
A large screening electrode reduces the maximum electric field strength in its vicinity. However, it causes a higher field strength at larger distances than a smaller electrode, which is often disturbing, since maximum field strength outside a device are often specified and must not be exceeded.
These conditions are illustrated in the simplest way by means of a model according to FIG. 1, where the screening electrode 1 is assumed to be a sphere with a radius r0 and at an electric potential U towards a distant ground plane 2. With these assumptions, the electric field strength at the distance r from the centre of the electrode is
E=U(r0/r2),
and it increases proportionally to the radius r0 of the electrode. The capacitance C of the electrode is
C=4xcfx80∈r∈0r0
where ∈r is the relative dielectric permittivity of the medium and ∈0 is the dielectric constant. Also the capacitance thus increases proportionally to the radius r0 of the electrode.
A typical embodiment of screening electrodes according to the state of the art is shown in FIG. 2, where a high-voltage apparatus 3, for example a capacitor, terminates in a toroidal screening electrode 1. For high voltages, composite screening electrodes are often used, a typical embodiment being shown in FIGS. 3a, 3b and 3c from EP patent specification EP 0 075 884 B1, where FIG. 3a shows a screening electrode 1 composed of 12 disc-shaped electrode segments 1a which are fixed to an icosahedron-shaped frame 5 composed of rods 6 according to FIG. 3b, with the fixing elements placed in a depression 4 in the electrode element to fix the electrodes to the icosahedron corners 7. An electrode segment 1a with the depression 4 is shown in cross section in FIG. 3c. A composite electrode according to EP 0 075 884 is lighter and simpler to manufacture than a corresponding electrode which is made up of a coherent electrode, but its dimensions have not be diminished.
When changing from two-dimensional electrode devices as, for example, GIS lines, where the inner conductor is here interpreted as an electrode, to three-dimensional configurations, for example line bends, the field strength increases since the curvature of the surface is larger for a sphere than for a cylinder with the same radius. FIG. 7 shows a 90xc2x0 line bend for GIS according to the state of the art. The cylinder-shaped inner conductor 10 has the radius rc, the tubular outer conductor/screen 11 has the radius Rc. In the actual line bend, the screen assumes the shape of a sphere 12 with the radius Rs and the inner conductor, the electrode, becomes a sphere 1 with the radius rs. For the line bend to be free of discharges, the following must apply, namely that, rs greater than rc and Rs greater than Rc; usually, the radii at the line bend are about 50% larger than in the straight lines.
It is also known to provide the electrodes with an electric non-conducting covering, for example to make difficult the emission of photoelectrons from the electrode surface, or as corrosion protection. However, these coverings are thin compared with the dimensions of the electrode, and their influence on the electric field around the electrode is therefore negligible. In this way, the dimensions of the electrode cannot be reduced.
An electrode for field control according to the invention solves the task of preventing breakdown or partial discharges in a gaseous insulating medium with reduced external dimensions and reduced capacitance.
An electrode for field control according to the invention comprises an inner electrode with an electrically conducing surface which is surrounded by a thick layer of an electrically insulating material with a low relative dielectric permittivity, preferably a polymeric foam containing gas bubbles or a matrix in which hollow gas-filled microspheres are embedded. By a thick layer is here meant a layer in which the ratio of the thickness d of the layer to the diameter 2r of the inner electrode is greater than 0.05, preferably greater than 0.15 and still more preferably 0.25. For non-spherical electrodes, the diameter 2r of the inner electrode is considered to be the mean diameter of the electrode. A sufficiently good approximation of the mean diameter is here the mean value of the diameter in two directions perpendicular to each other. Inner electrode here also means an electrode element in a composite electrode. To achieve the effect of the invention, only those parts of the inner electrode, where a considerable increase of the electric field strength occurs, need to be covered with the non-conducting layer. Preferably, at least that one-third of the surface of the inner electrode is covered which, without covering with the layer, would exhibit the highest field strength.
The mode of operation of the invention can be most readily illustrated by means of a model according to FIG. 4, in which the inner electrode is assumed to be sphere 1xe2x80x2 with the radius r1 and at an electric potential U towards a distant ground plane 2, where the inner electrode is surrounded by a concentric layer 8 of electrically insulating material with a thickness of r2xe2x88x92r1 where r2 is the outer radius of the insulating layer and 9 its outer edge. For the sake of simplicity, the insulating material is assumed to have such a high proportion of internal cavities, as gas bubbles or hollow microspheres, that the relative dielectric constant of the material is ∈r≈1, as is the dielectric constant of the gaseous medium. Since the diameters of the inner cavities are chosen so small that the dielectric strength of the gas contained in the cavity according to Paschen""s law is considerably greater than the dielectric strength of a macroscopic gap within the same gas, this insulating material has a considerably greater dielectric strength than the gas surrounding the electrode. Typical mean diameters are smaller than 300 micrometres. For a more detailed analysis of dielectric strengths in polymeric foams, reference is made to an article xe2x80x9cBreakdown of Polyurethane Hard-foam Insulation under Short-time HV Stressxe2x80x9d by D. Koenig, B. Bayer and H. J. Heller in IEEE Transactions on Electrical Insulation, Vol. 24, No. 2, April 1989, pages 239-248. In this case with ∈r≈1, the same relations apply to the field strength at the distance r from the centre of the electrode and the capacitance as to an electrically conducting sphere with the radius r1 at potential U towards a distant ground plane as shown under the discussion of the background art:
E=U(r0/r2)C=4xcfx80∈r∈0r0
It can now be shown that, for an insulating medium with a dielectric strength which is kxc2x7E0 with E0 as the dielectric strength of the gaseous medium, an electrode according to the invention may be designed where the field strength at the surface of the inner electrode falls below the dielectric strength kxc2x7E0 of the insulating material, and the field strength at the outer surface of the insulating layer falls below the dielectric strength E0 of the gaseous medium if the conditions
r1 greater than r0/k and r2 greater than r0/k
are fulfilled, where r0 is the radius of an electrode according to the prior art (electrically conducting sphere) where the dielectric strength E0 of the gaseous medium is attained at the surface of the electrode. For example, the radius of the inner electrode is reduced, and hence also the capacitance, by a factor of four and the outer radius of the insulating layer is reduced by a factor of two if k=4.
If the relative dielectric constant of the insulating material is ∈r greater than 1, the possible outer radius r2 becomes somewhat larger. When maintaining the ratio r2/r1=2, as in the example above, the following applies instead
r2=r0/(1+∈rxe2x88x921)
For ∈r=1.3, a typical value of a foamed plastic, thus
r2=0.57 r0,
which is still a considerable reduction of the size of the electrode. For the insulating layer to have a noticeable effect according to the invention, its relative dielectric constant ∈r should be smaller than 3, preferably smaller than 2 and still more preferably smaller than 1.5
An electrode according to the invention is preferably used in electric equipment for voltages exceeding 1 kV.